(CNN) -- You think you're being smart when, in an effort to eat more healthfully, you check a restaurant's website to see how many calories are in a dish you plan to order.

It turns out perhaps that effort isn't worth as much as you think. A new study by Tufts University nutrition researchers shows nearly one out of five restaurant dishes has at least 100 more calories than what a restaurant states on its website.

The underestimated foods came from several restaurant chains, including Chipotle Mexican Grill, Olive Garden, Boston Market and Outback Steakhouse.

"I think restaurants have a lot to answer for here," said Susan Roberts, senior author of the paper published in this week's Journal of the American Medical Association.

The study looked at 269 food items at 42 fast-food and sit-down restaurants in Massachusetts, Arkansas and Indiana between January and June 2010. Lead author Lorien Urban, then a Tufts graduate student, and others purchased the foods without telling the restaurants they were for a study. They brought the foods to the lab at Tufts, analyzed the calories and then compared them with the calories listed at that time on the restaurants' websites.

On average, the calorie counts were accurate. However, the lab analysis showed that 19% of the foods tested had 100 or more calories in excess of what was on the website.

"One food had more than 1,000 calories more than it was supposed to," Roberts said, referring to a side order of chips and salsa at On the Border Mexican Grill & Cantina. "It was just shocking."

Sit-down restaurants were more likely to have inaccurate readings. Roberts said she thinks this is because individual workers have some leeway in how they prepare the foods, whereas the process is more automated in fast food restaurants.

In fact, several restaurants have caveats such as the one on the Outback Steakhouse website, which states that "menu items are hand-prepared and caloric values may vary from the stated amount."

A new federal law will require calories to be stated on menus at large chain restaurants in the next year. Due to this new rule, the National Restaurant Association said, "Many restaurant chains are looking at tighter kitchen quality control standards."

The Tufts researchers looked at information on the restaurants' websites, which listed the number of calories absorbed by the body, which is different from the total number of calories in a dish. Using information from the restaurants' websites, the researchers estimated the total number of calories in the food and then tested the food in the lab and made a comparison.

According to the Tufts lab analysis, Chipotle's burrito bowl with rice, black beans, peppers, onions, lettuce, green tomatillo salsa and cheese had 703 total calories -- 249 more than what was expected based on information from the restaurant's website.

In a statement, Chipotle acknowledged there could be calorie differences between what's posted on its website and what's served to customers because of "the seasonality of ingredients, adherence to recipes, and cooking from scratch."

At Olive Garden, Tufts found the chicken and gnocchi soup had 529 calories, which was 246 more than what would be expected based on the restaurant's website.

Olive Garden told CNN the numbers on its website "are as accurate as they can be for dishes that are individually crafted by hand."

At Boston Market, three pieces of dark meat chicken (two drumsticks and a thigh) had 572 calories, according to the lab analysis -- 215 calories more than what would be expected from the restaurant's nutrition information. Boston Market did not respond to CNN's request for comment.

Some foods with the biggest discrepancies were lower-calorie items such as salads, which dieters would be more likely to choose. For example, the Tufts lab analysis showed the classic blue cheese wedge side salad at Outback Steakhouse contained 1,035 calories -- 659 calories more than what would be expected based on what was on the restaurant's website.

Joe Kadow, executive vice president of OSI Restaurant partners, which owns Outback Steakhouse, said he suspected the Tufts researchers tested an entrée-size salad rather than a side salad.

Urban said she's sure she ordered the side salad.

"Looking at it, I think the issue is they put on a lot of dressing -- more than they normally put," she added.

Roberts, author of "The 'I' Diet," said these discrepancies help explain why some people in her diet groups have had trouble losing weight even when they used a restaurant's website to select lower calorie dishes.

"If you have 100 calories more than you think (every day), that's something like 10 or 15 pounds of extra weight you gain over the course of the year," she said.

She advises dieters to order items such as dressing, cheese and sauce on the side, so they can have better control over calories.